1. Field of the Invention
The present invention relates to a multi-layered optical film, and more particularly a multi-layered optical film with excellent stability of optical characteristics in a function of time.
2. Related Background Art
In optical elements, the surface reflection reduces the transmittance of the optical system and increases the light not contributing to the imaging, thereby lowering the image contrast. For this reason, an antireflection film is provided on the surface of various optical elements, in order to reduce the surface reflection. Also in the field of optical elements there is already known a beam splitter (light intensity splitting element) for splitting the incident light into a reflected light and a transmitted light with a desired ratio of light intensities. Such optical thin film is usually formed as evaporated films based on metal oxides, and is laminated as a combination of films of a low refractive index and those of a high refractive index. The metal oxide constituting each layer of such multi-layered optical thin film is required to have a desired refractive index, to be optically homogeneous, to be free from absorption thus showing excellent transparency, and to be stable in optical characteristics over time.
For the high refractive index film in such multi-layered optical film, there is widely employed an evaporation film, composed of at least a component selected from a group consisting of TiO.sub.2, Ta.sub.2 O.sub.5, ZrO.sub.2, HfO.sub.2, Nb.sub.2 O.sub.5, mixtures thereof and modified substances thereof. Also for the low refractive index film there is employed SiO.sub.2 film in consideration of the refractive index and other properties.
However, such multi-layered optical film, formed with the high refractive index films composed of at least a component selected from a group consisting of TiO.sub.2, Ta.sub.2 O.sub.5, ZrO.sub.2, HfO.sub.2, Nb.sub.2 O.sub.5, mixtures and modified substance thereof, and with the low refractive index films composed of SiO.sub.2, has been associated, when deposited on a substrate of which temperature cannot be made sufficiently high at the film formation, such as a resin substrate, with drawbacks of shift of optical characteristics toward the longer wavelength side over time, and of decrease of the effect of the multi-layered film, due to significant changes as a function of time in the refractive index of the component films.